1. Field of the Invention
The invention relates to metal oxides surface-modified with a fraction of at least 15% of T1 groups or a fraction of T1 groups which is greater than the fraction of T3 groups, where the fraction of the modified metal oxides that is extractable with solvents is not more than 15 wt %, and also to a method for producing these metal oxides and to their use as a rheological additive for adjusting the viscosity, yield point, and the shear-thinning and also thixotropic properties in adhesives, sealants, and coating materials such as paints, inks, and varnishes, for example.
2. Description of the Related Art
Particulate metal oxides with organic functionality are commonly used as active fillers for enhancing the mechanical properties of materials which find diverse use. It is possible by this means, for example, to enhance the scratch resistance of coatings such as paints, inks, and varnishes, or to tailor the mechanical properties of adhesives and sealants. Furthermore, however, even in the uncrosslinked state of the matrix systems, the nanostructured fillers fulfill an extremely important function. Thus the dispersions frequently have relatively high viscosities and in many cases in fact have viscoelastic properties. This pseudoplastic behavior plays an important part in particular for the operation of processing the materials in question. Through deliberate chemical surface modification of the particulate fillers it is possible to steer the interactions with the matrix surrounding them and hence to control the viscoelastic behavior of the dispersion.
WO 2008/077814 discloses the production of resin layers with organic functionality on particulate metal oxides of high specific surface area, and discloses the modified particulate metal oxides and their use. A feature of the metal oxides disclosed is a defined silicone resin structure with organic functionality, a spectroscopic analysis of which is given. In examples 1 to 5, the sum of the intensities of the T2 and T3 groups is greater by a factor of at least 5 than the intensity of the T1 groups; in other words, the fraction of T1 groups on the surface of the modified metal oxide, based on the sum of T1+T2+T3, is in all cases below 15% and is lower than the fraction of T3 groups. Correspondingly, the metal oxides are notable for highly crosslinked silicone resin layers and a low thickening effect in a liquid medium. The metal oxides disclosed in specification WO 2008/077814 are therefore particularly suitable for achieving high levels of filling.
Conversely, in comparative examples 6 and 7 of WO 2008/077814, a higher fraction of T1 groups is measured in relation to the sum of all T groups, and for these metal oxides the extractable fraction is high, at 19.5 wt % and 25.1 wt %, respectively. The method described operates with a high deployment of covering agent, as calculated from the amount of covering agent substance used, relative to the surface area of the silica, and being situated within the region of 9 μmol/m2. The reaction products resulting from this method, with a low degree of condensation, have a considerable fraction of silicon-containing compounds which in chemical terms are not firmly attached, and this presents a problem for the further use of the surface-treated metal oxides as, for example, in their use as additives for rheology control.